This invention relates to a suspension system for a vehicle and more particularly to a suspension system which comprises a shock absorber of damping force characteristic variable type provided between a sprung member and an unsprung member.
In general, a suspension system for a vehicle comprises a shock absorber, for damping the oscillation of a wheel, interposed between a sprung member and an unsprung member. There are various types of shock absorbers and shock absorbers of damping force characteristic variable type are grouped into one which damping force characteristic (characteristic of different damping coefficient) is changed into two levels (higher level and lower level) and the another which the damping force characteristic is changed into many levels or changed without steps.
A well known shock absorber of damping force characteristic variable type is, as disclosed in the Japanese Patent Application Laying Open Gazette No. 61-163011, is to detect absolute speed of the sprung member and relative speed between the sprung member and the unsprung member by each detecting means and check signs of them. If these signs are not same, it is observed that the damping force generated by the shock absorber is acting on the oscillation-stimulating direction with respect to the vertical oscillation of a vehicle and set the damping force characteristic of a shock absorber lower (i.e., SOFT side). When these signs are same, it is is observed that the damping force is acting on oscillation-restricting direction and set the damping force characteristic of the shock absorber higher (i.e., HARD side). Thus, passenger riding comfort and running stability is improved.
The Japanese Utility Model Registration Application Laying Open Gazette No. 61-110412 and 63-40213 disclose a shock absorber in which an insensible range is formed near neutral position with respect to the relative displacement between the sprung member and the unsprung member in order to prevent the frequent switching of the damping force characteristic of the shock absorber near the neutral position of that displacement. In this insensible range, switching of the damping force characteristic of the shock absorber is restricted and lower damping force characteristic is maintained.
Maintaining the lower damping force characteristic of the shock absorber in the insensible range is effective when the sprung member is in high oscillation frequency region due to the road bumps. However, if lower damping force characteristic of the shock absorber is maintained when the sprung member is in low oscillation frequency range, running stability is not be satisfied.
In order to solve the above problem, a method may be taken for detecting the oscillation frequency of the sprung member by a sensor so that the damping force characteristic in the insensible range is set lower during high oscillation frequency region and the damping force characteristic in the insensible range is set higher during low oscillation frequency region. However, this method requires a sensor for detecting the oscillation frequency and results in higher cost.
On the other hand, when considering the absolute value of the absolute speed of the sprung member and absolute value of either the relative displacement between the sprung member and the unsprung member or the relative speed between the sprung member and the unsprung member with respect to oscillation frequency, .vertline.zs.vertline. which is the absolute value of the absolute speed of the sprung member is, as shown in FIG. 6, kept higher during oscillation frequency is lower than the resonance point .omega.2 (near 10.about.20 Hz) of the unsprung member and decreased during oscillation frequency is higher than the resonance point .omega.2 of the unsprung member. Also, each .vertline.zs-zu.vertline. which is the absolute value of the relative speed between the sprung member and the unsprung member and .vertline.zs-zu.vertline. which is the absolute value of the relative displacement between the sprung member and the unsprung member is, as shown in FIG. 7, maximum at both the resonance point .omega.1 (near 1.0.intg.2.0 Hz) of the sprung member and the resonance point .omega.2 of the unsprung member and decreases during the oscillation frequency is lower than the resonance point .omega.1 of the sprung member and during the oscillation frequency is higher than the resonance point .omega.2 of the unsprung member.